Electrostatic lens structure

ABSTRACT

An electrostatic lens comprising a first conductive plate with a first aperture, a second conductive plate with a second aperture, the second aperture being substantially aligned with the first aperture, a voltage supply for supplying a first voltage to the first conductive plate and a second voltage to the second conductive plate, the first voltage being lower than the second voltage, and an insulating structure for separating the first conductive plate from the second conductive plate. The insulating structure comprises a first portion in contact with the first conductive plate and a second portion in contact with the second conductive plate, the first portion having an overhanging portion and the second portion having an indented portion at an edge of the insulating structure, so that a gap is formed between the overhanging portion and the second conductive plate.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an electrostatic lens or an electrostatic lensarray. Furthermore, the invention relates to a charged particlelithography system comprising such an electrostatic lens orelectrostatic lens array. Additionally, the invention relates to amethod of manufacturing an insulating structure for use in anelectrostatic lens.

2. Description of the Related Art

Electrostatic lenses are used in charged particle beam columns for usein applications like scanning electron microscopes and lithographyapparatus. An electrostatic lens typically consists of conductiveplates, generally silicon plates provided with a conductive layer,stacked in a direction of the charged particle beam. The conductiveplates may be separated by electrically isolating spacers, e.g. made ofglass or other suitable insulating materials. The plates and spacershave aligned apertures enabling a charged particle beam to pass throughthe plates along a charged particle transmission path. The plates may becharged to enable actions such as focusing, blanking and steering of thecharged particle beam.

The electric field that can be applied between the plates depends on theplate material properties and the positioning of the plates with respectto each other. Furthermore, the properties and structure of theinsulating barrier between the plates, e.g. electrically isolatingspacers made of glass or vacuum, are of importance. If the insulatingbarrier is stressed beyond its dielectric strength, a sudden transitionfrom an insulating state to a highly conductive state takes place. Sucha transition may be in the form of an electric discharge or flashover.

The surface of a solid insulator will often provide a path for such anelectrical discharge. If such a discharge occurs in a solid dielectric,typically occurring along a surface of the insulator between the plates,changes along a discharge path caused by the discharge can result in apermanent reduction in the dielectric strength of the insulator. As aresult, the insulator is degraded and a high electric field cannot bemaintained by the electrostatic lens, reducing the performance of thelens.

Conventional efforts to reduce such electrical discharges betweenconductive plates rely on increasing the length of the breakdown pathalong surfaces where electrical breakdown is likely to occur, ascompared to the direct, i.e. shortest distance between the plates.Examples of this approach are described in Japanese patent publicationsJP2000260242 and JP2005214908, where beveled or rectangular recesses areincluded at the location of connection with an electrode.

However, the requirement to maintain the electrode plates close togetherlimits the ability to elongate the discharge path between theelectrodes. Furthermore, merely increasing the surface breakdown pathlength may be insufficient if high electric fields need to be appliedbetween conductive plates positioned in close proximity to each other,e.g. resulting in 10-50 V/micron of electric field to be covered.Therefore, further design measures are needed to enable electrostaticlenses to operate under these circumstances without a considerable riskof electrical breakdown.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an electrostatic lens orelectrostatic lens array with an improved performance with respect toelectrical breakdown as compared to the state of the art.

The insight behind the invention includes that the flashover occurs, orat least starts, by the extraction and emission of electrons from one ofthe conductive (electrode) plates, caused by the electric field.Furthermore, the extraction of electrons will be higher at a negativeelectrode than at a positive electrode, and thus the electric fieldshould be kept low particularly at the negative electrode.

It has also been recognized that electric field enhancement plays a rolein causing flashover and recognition of this leads to better methods forpreventing flashover. Any enhancement of the electric field should beavoided or reduced, particularly at the negative electrode. Voidsbetween a portion of the insulating structure and one of the conductiveplates cause an enhancement or elevation of the electric field locallyin the void. The smaller the width of the void in comparison to thedistance between the plates, the greater will be any enhancement of theelectric field in the void. Such an electric field enhancement makes thelocation of the void more susceptible to flashover. When such a voidoccurs at a so-called triple points, i.e. a point at which there is aboundary between air/vacuum, conductor and insulator, the likelihood ofan electrical discharge increases considerably. This is particularly thecase where the triple point occurs adjacent to a more negative or lowervoltage conductive element. Such voids at triple points may occur inpractice due to chipping or cracking at the edge of the insulator or asa result of manufacturing errors or tolerances causing surfaceirregularities during fabrication of the conductive plates or insulatingstructures.

The invention seeks to provide measures to limit the electric fieldenhancement, particularly at triple points, and particularly adjacent toa more negative or lower voltage electrode in a electrostatic lens orelectrostatic lens array, to permit the lens or lens array to withstandhigh electric fields while reducing the occurrence of electricaldischarges. These measures may be used to avoid the necessity ofadopting path elongation measures.

For this purpose, according to one aspect of the invention, anelectrostatic lens includes a first conductive plate with a firstaperture, a second conductive plate with a second aperture, the secondaperture being substantially aligned with the first aperture, a voltagesupply for supplying a first voltage to the first conductive plate and asecond voltage to the second conductive plate, the first voltage beinglower than the second voltage, and an insulating structure forseparating the first conductive plate from the second conductive plate.The insulating structure comprises a first portion in contact with thefirst conductive plate and a second portion in contact with the secondconductive plate, the first portion having an overhanging portion andthe second portion having an indented portion at an edge of theinsulating structure, wherein a gap is formed between the overhangingportion and the second conductive plate.

By this measure, any electric field enhancement in a void at the edge ofthe insulating structure between the first portion and the firstconductive plate can be reduced. This field enhancement will bedistributed over the small void and the larger gap below the overhangingportion of the insulating material where the void occurs. Since the voidis typically very small and the gap is typically much larger, the fieldenhancement will be considerably reduced in the void than when the fieldwas concentrated solely therein. The location of the (large) gapstructure at the positive or higher voltage electrode further increasesthe beneficial effect, because in this way the harmful effects arereduced of any voids at the negative or lower voltage electrode that canmost easily result in a flashover.

The permittivity of the gap is preferably higher than the permittivityof the insulating structure, preferably at least four times higher. Inoperation, the electric field strength in the gap between theoverhanging portion of the insulating structure and the secondconductive plate is preferably greater than the electric field strengthacross the second portion of the insulating structure. The first portionand the second portion of the insulating structure may be made of equalthickness, and the first portion and the second portion compriseseparate structures which are bonded together.

The surface of the insulating structure facing the first conductiveplate may be provided with a conductive layer or coating in electricalcontact with the first conductive plate for limiting electric fieldenhancement between the first conductive plate and the insulatingstructure. The conductive layer may be deposited on a surface of theinsulating structure using a deposition technique. The conductive layermay comprise chromium or tantalum, and the insulating structure maycomprise borosilicate glass.

The inclusion of a conductive coating on the surface of the insulatingstructure results in any voids or irregularities on the surface of theinsulating structure being covered by the conductive material, which isin electrical contact with the first conductive plate. These surfaceirregularities, which would have formed field enhancing voids againstthe surface of the first conductive plate, will have the same surfacepotential as the first conductive plate, so that there is no electricfield in the void. This effectively prevents the occurrence of anenhanced electric field in these voids.

The distance between the first conductive plate and the secondconductive plate preferably lies in a range from about 100 to 200micrometers, and the lens is preferably capable of withstanding anelectric field strength within a range from about 10 to 25 volts permicron, and even 25 to 50 volts per micron without flashover occurring.

The invention in another aspect also includes an electrostatic lensarray comprising a plurality of electrostatic lenses as describedherein. The insulating structure of this electrostatic lens array maytake the form of an insulating plate provided with at least one thirdaperture, the third aperture being arranged such that a projection ofits side walls circumscribes multiple first apertures and secondapertures. The insulating structure may also take the form of aplurality of elongated bars, such that between successive bars multipletransmission paths formed by first apertures and second apertures arepresent.

The invention according to another aspect comprises a charged particlebeamlet lithography system including a source of charged particles forproducing a beam of charged particles, an aperture array for producing aplurality of beamlets from the charged particle beam, a beamletconditioning system for conditioning the plurality of beamlets inaccordance with a pattern, and the electrostatic lens array asddescribed herein for focusing the plurality of beamlets. Theelectrostatic lens array may be positioned between the aperture arrayand the beamlet conditioning system. The charged particle beamletlithography system may also include a support unit for supporting asubstrate to be patterned and the electrostatic lens array positioneddownstream of the beamlet conditioning system for focusing a pluralityof conditioned beamlets on a target surface of the substrate.

Yet another aspect of invention relates to a method of manufacturing aninsulating structure for use in an electrostatic lens, the methodcomprising providing a first insulating plate, manufacturing at leastone first through hole in the first insulating plate, the at least onefirst through hole having a first diameter, providing a secondinsulating plate, manufacturing at least one second through hole in thesecond insulating plate, the at least one second through hole having asecond diameter, the second diameter being larger than the firstdiameter, and combining the first insulating plate and the secondinsulating plate to form an insulating structure such that theinsulating structure has at least one aperture formed by the at leastone first and second through holes.

The first and second insulating plates may be made of the sameinsulating material, and at least one of the plates may compriseborosilicate glass. Manufacturing of the first and/or second throughholes may be performed by power blasting.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the invention will be appreciatedupon reference to the following drawings, in which:

FIG. 1 is a simplified schematic drawing of an example of a chargedparticle multi beamlet lithography system;

FIG. 2 is a simplified schematic drawing, in side view, of an end moduleof the lithography system of FIG. 1;

FIGS. 3A-3C schematically depict three electrostatic lens arrangementswhere field enhancement may occur;

FIG. 4 schematically shows a side view of a portion of an electrostaticlens array according to a first aspect of the invention;

FIG. 5 schematically shows a side view of a portion of an electrostaticlens or lens array according to a second aspect of the invention;

FIG. 6A schematically shows a side view of a portion of an electrostaticlens or electrostatic lens array according to a third aspect of theinvention;

FIG. 6B shows a detail of the electrostatic lens or electrostatic lensarray of FIG. 6a;

FIG. 7 schematically shows a side view of a portion of an electrostaticlens array combining different aspects of the invention;

FIG. 8A schematically shows a top view of an electrostatic lens arrayaccording to an embodiment of the invention;

FIG. 8B schematically shows a side view of a portion of theelectrostatic lens array of FIG. 8A.

DESCRIPTION OF SPECIFIC EMBODIMENTS

The following is a description of a number of embodiments of theinvention, given by way of example only and with reference to thedrawings. FIG. 1 schematically shows a simplified schematic drawing ofan example of a charged particle multi-beamlet lithography system basedupon an electron beam optical system without a common cross-over of allthe electron beamlets. Such lithography systems are described forexample in U.S. Pat. Nos. 6,897,458, 6,958,804, 7,084,414, and7,129,502, which are all hereby incorporated by reference in theirentirety, assigned to the owner if the present invention.

The lithography system comprises an electron source 1 for producing ahomogeneous, expanding electron beam 20. Beam energy is preferablymaintained relatively low in the range of about 1 to 10 keV. To achievethis, the acceleration voltage is preferably low, the electron sourcepreferably kept at between about −1 to −10 kV with respect to the targetat ground potential, although other settings may also be used.

The electron beam 20 from the electron source 1 passes a double octopole2 and subsequently a collimator lens 3 for collimating the electron beam20. Subsequently, the electron beam 20 impinges on an aperture array 4,which blocks part of the beam and allows a plurality of beamlets 21 topass through the aperture array 4. The aperture array preferablycomprises a plate having through holes. Thus, a plurality of parallelelectron beamlets 21 is produced. The system generates a large number ofbeamlets 21, preferably about 10,000 to 1,000,000 beamlets, although itis of course possible to use more or less beamlets. Note that otherknown methods may also be used to generate collimated beamlets.

The plurality of electron beamlets 21 pass through a condenser lensarray 5 which focuses each of the electron beamlets 21 in the plane of abeam blanker array 6. This beamlet blanker array 6 preferably comprisesa plurality of blankers which are each capable of deflecting one or moreof the electron beamlets 21.

Subsequently, the electron beamlets 21 enter the end module 7. The endmodule 7 is provided with a plurality of openings to accommodate theplurality of electron beamlets 21. In one embodiment, each beamlet 21passes through its own opening, while in another embodiment a group ofbeamlets 21, converging near an opening, will pass through that opening.The end module 7 is preferably constructed as an insertable, replaceableunit which comprises various components. The end module 7 comprises abeam stop array 8, a beam deflector array 9, and a projection lensarrangement 10, although not all of these need be included in the endmodule 7 and they may be arranged differently. The end module 7 will,amongst other functions, provide a demagnification of about 50 to 500times. High values within this range enable r less stringentmanufacturing requirements in the upper part of the electron beamlithography system. Furthermore, such high values may enable a reductionof the height of the system. After leaving the end module 7, thebeamlets 21 impinge on a surface of a target 11 positioned at a targetplane. For lithography applications, the target usually comprises awafer provided with a charged-particle sensitive layer or resist layer.

In the end module 7, the electron beamlets 21 first pass beam stop array8. This beam stop array 8 largely determines the opening angle of thebeamlets. The beam stop array, in its basic form, comprises a substrateprovided with through holes for allowing beamlets to pass through.

The beamlet blanker array 6 and beam stop array 8 operate together toblock or let pass the beamlets 21. The passages of the beam stop array 8may be aligned with the elements of the beam blanker array 6. In such acase, if beamlet blanker array 6 deflects a beamlet, the beamlet willnot pass through the corresponding aperture in beam stop array 8, butinstead will be blocked. On the other hand, if beamlet blanker array 6does not deflect a beamlet, the beamlet will pass through thecorresponding aperture in beam stop array 8 and will then be projectedas a spot on the surface of target 11.

Next, the beamlets pass through a beam deflector array 9 which providesfor deflection of each beamlet 21 in the X and/or Y direction,substantially perpendicular to the direction of the undeflected beamlets21. Next, the beamlets 21 pass through projection lens arrangement 10and are projected onto a target 11, typically a wafer, in a targetplane.

For consistency and homogeneity of current and charge both within aprojected spot and among the projected spots on the target, the diameterof the apertures in beam stop array 8 are preferably smaller than thediameter of the beamlets when they reach the beam stop array.

The diameter of the apertures in beam stop plate 8 in the presentexample limit the cross section of a beamlet. In this way, only acentral part of a beamlet is allowed to pass through beam stop plate 8for projection onto target 11. This central part of a beamlet has arelatively uniform charge density. Furthermore, if the apertures in beamstop array 8 are round, the beamlets passing through the beam stop plate8 will have a generally uniform opening angle.

FIG. 2 shows an embodiment of end module 7 in more detail, showing thebeam stop array 8, the deflection array 9, and the projection lensarrangement 10, projecting an electron beamlet onto a target 11. Thebeamlets 21 are projected onto target 11, preferably resulting in ageometric spot size of about 10 to 30 nanometers in diameter. Theprojection lens arrangement 10 in such a design preferably provides ademagnification of about 100 to 500 times.

The projection lens arrangement 10, in the embodiment shown in FIG. 2,has three plates 12, 13 and 14 arranged in sequence, used to form anarray of electrostatic lenses. The plates 12, 13, and 14 preferablycomprise substrates with apertures formed in them. The apertures arepreferably formed as round holes though the substrate, although othershapes can also be used. The substrates may be formed of silicon oranother semiconductor processed using process steps well-known in thesemiconductor chip industry. The apertures can be conveniently formed inthe substrates using lithography and etching techniques known in thesemiconductor manufacturing industry, for example. The lithography andetching techniques used are preferably controlled sufficiently preciselyto ensure uniformity in the position, size, and shape of the apertures.

For example, apertures in plates 12, 13, and 14, if they are made ofsilicon, may be formed by using a so-called Bosch etching process. TheBosch etching process is a method for anisotropically etching silicon.More details with respect to such a Bosch etching process may be foundin European patent 0 625 285 assigned to Robert Bosch GmbH.

A uniform distance (pitch) between the apertures and uniform arrangementof the apertures over the surface of the substrate permits theconstruction of a system with densely packed beamlets which generate auniform grid pattern on the target. The pitch between the apertures maybe in the range 50 to 500 microns with a deviation in the pitch ofpreferably 100 nanometers or less. Furthermore, in systems wheremultiple plates are used, the corresponding apertures in each plate arealigned. Misalignment in the apertures between plates may cause adifference in focal length along different axes.

Uniformity in the size of the apertures enables uniformity in theelectrostatic projection lenses formed at the locations of theapertures. The size of the apertures may be in the range of 50 to 150microns with a deviation in the size of preferably 100 nanometers orless.

Uniformity in the shape of the apertures is also preferred. Where roundholes are used, uniformity in the roundness of the holes results in thefocal length of the resulting lens being the same all round.

The substrates may be coated in an electrically conductive coating toform electrodes. The conductive coating preferably forms a singleelectrode on each substrate covering both surfaces of the plate aroundthe apertures and inside the holes. A metal with a conductive nativeoxide is preferably used for the electrode, such as molybdenum,deposited onto the plate using techniques well known in thesemiconductor manufacturing industry, for example. An electrical voltageis applied to each electrode to control the shape of the electrostaticlenses formed at the location of each aperture. Each electrode ispreferably controlled by a single control voltage for the completearray. Thus, in the embodiment shown in FIG. 2 with three electrodeslens there will be only three voltages for all the thousands of lenses.

In FIG. 2, the plates 12, 13, and 14 have electric voltages V1, V2 andV3 respectively applied to their electrodes. The voltage differencesbetween the electrodes of plates 12 and 13, and between plates 13 and14, create electrostatic lenses at the location of each aperture in theplates. This generates a “vertical” set of electrostatic lenses at eachposition in the array of apertures, mutually aligned, creating an arrayof projection lens systems. Each projection lens system comprises a setof electrostatic lenses formed at corresponding points of the arrays ofapertures of each plate. Each set of electrostatic lenses forming aprojection lens system can be considered as a single effectiveprojection lens, which focuses and demagnifies one or more beamlets, andhas an effective focal length and an effective demagnification. Insystems where only a single plate is used, a single voltage may be usedin conjunction with a ground plane, such that electrostatic lenses areformed at the location of each aperture in the plate.

Variation in the uniformity of the apertures will result in variation inthe electrostatic lenses forming at the locations of the apertures. Theuniformity of the apertures results in uniform electrostatic lenses.Thus, the three control voltages V1, V2, and V3 create an array ofuniform electrostatic lenses which focus and demagnify the large numberof electron beamlets 21.

The characteristics of the electrostatic lenses are controlled by thethree control voltages, so that the amount of focusing anddemagnification of all of the beamlets can be controlled by controllingthese three voltages. In this way, a single common control signal can beused to control a whole array of electrostatic lenses for demagnifyingand focusing a very large number of electron beamlets. A common controlsignal may be provided for each plate or as a voltage difference betweentwo or more plates.

The number of plates used in different projection lens arrangements mayvary, and the number of common control signals may also vary. Where theapertures have sufficiently uniform placement and dimensions, thisenables the focusing of the electron beamlets, and demagnification ofthe beamlets, using one or more common control signals. In theembodiment of FIG. 2, three common signals comprising the three controlvoltages V1, V2, and V3 are thus used to focus and demagnify all of thebeamlets 21.

The projection lens arrangement preferably forms all of the focusingmeans for focusing the beamlets onto the target surface. This is madepossible by the uniformity of the projection lenses, which providesufficiently uniform focusing and demagnification of the beamlets sothat no correction of the focus and/or path of individual electronbeamlets is required. This considerably reduces the cost and complexityof the overall system, by simplifying construction of the system.simplifying control and adjustment of the system, and greatly reducingthe size of the system.

Preferably, the placement and dimensions of the apertures where theprojection lenses are formed are controlled within a tolerancesufficient to enable focusing of the electron beamlets using one or morecommon control signals to achieve a focal length uniformity better than0.05%. The projection lens systems are spaced apart at a nominal pitch,and each electron beamlet is focused to form a spot on the surface ofthe target. The placement and dimensions of the apertures in the platesare preferably controlled within a tolerance sufficient to achieve avariation in spatial distribution of the spots on the surface of thetarget of less than 0.2% of the nominal pitch.

The projection lens arrangement 10 may be compact with the plates 12,13, 14 being located close to each other, so that despite the relativelylow voltages used on the electrodes (in comparison to voltages typicallyused in electron beam optics), it can produce very high electricalfields. These high electrical fields generate electrostatic projectionlenses which have a small focal distance, since for electrostatic lensesthe focal length can be estimated as proportional to beam energy dividedby electrostatic field strength between the electrodes. In this respect.where previously 10 kV/mm could be realized, potential differences ofover 10 kV/mm, or even in the range 25 to 50 kV/mm may be providedbetween the second plate 13 and third plate 14.

These voltages V1, V2, and V3 are preferably set so that the differencein voltage between the second and third plates (13 and 14) is greaterthan the difference in voltage between first and second plates (12 and13). This results in stronger lenses being formed between plates 13 and14 so that the effective lens plane of each projection lens system islocated between plates 13 and 14, as indicated in FIG. 2 by the curveddashed lines between plates 13 and 14 in the lens opening. This placesthe effective lens plane closer to the target and enables the projectionlens systems to have a shorter focal length. It is further noted that,for simplicity, the beamlet in FIG. 2 is shown focused as from thedeflector 9.

The electrode voltages V1, V2, and V3 are preferably set so that voltageV2 is closer to the voltage of the electron source 1 than is voltage V1,causing a deceleration of the charged particles in beamlet 21. Forexample, if the target is at 0V (ground potential) and the electronsource is at about −5 kV relative to the target, voltage V1 may be about−4 kV, and voltage V2 may be about −4.3 kV. Voltage V3 then may be atabout 0V relative to the target, which avoids a strong electric fieldbetween plate 14 and the target 11. Such a strong electric field cancause disturbances in the beamlets if the topology of the target is notflat. The distances between the plates (and other components of theprojection system) are preferably small.

With this arrangement, a focusing and demagnifying projection lens isrealized, as well as a reduction in the speed of extracted chargedparticles in the beamlets. With the electron source at a voltage ofabout −5 kV, charged particles are decelerated by the central electrode(plate 13), and subsequently accelerated by the bottom electrode (plate14) having a voltage at ground potential. This deceleration permits theuse of lower electrical fields on the electrodes while still achievingthe desired demagnification and focusing for the projection lensarrangement.

FIG. 2 also illustrates deflection of a beamlet 21 by deflection array 9in the Y-direction, illustrated in FIG. 2 as a deflection of the beamletfrom left to right. In the embodiment of FIG. 2, an aperture indeflection array 9 is shown for one or more beamlets to pass through,and electrodes are provided on opposite sides of the aperture, theelectrodes provided with a voltage +V and −V. Providing a potentialdifference over the electrodes causes a deflection of the beamlet orbeamlets passing though the aperture. Dynamically changing the voltages(or the sign of the voltages) will allow the beamlet(s) to be swept in ascanning fashion, here in the Y-direction.

The arrangement of the deflectors and lenses of the end module 7 withrespect to one another as described differs from what has generally beenexpected in the art of particle optics. Typically, a deflector islocated after a projection lens, so that the focusing is accomplishedfirst and then the focused beamlet is deflected. First deflecting abeamlet and then focusing it, as in the system in FIG. 2, results in thebeamlet entering the projection lens off axis and at an angle withrespect to the optical axis of the projection lens. It is evident to theperson skilled in the art that the latter arrangement may give rise tooff-axis aberrations in the deflected beamlet.

In the lithography applications, a beamlet should be focused andpositioned at ultra high precision, with spot sizes of tens ofnanometers, with an accuracy in size of nanometers, and a positionaccuracy in the order of nanometers. The inventors realized thatdeflecting a focused beamlet, for example several hundreds of nanometersaway from the optical axis of a beamlet, would easily result in anout-of-focus beamlet. In order to meet the accuracy requirements, thiswould severely limit the amount of deflection or the beamlet wouldrapidly become out of focus at the surface of target 11.

As discussed above, in order to achieve the objectives of the projectionlens arrangement in view of its use in a lithography system, theeffective focal length of the projection lens systems is short, and thelens plane of the projection lens systems is positioned very close tothe target plane. Thus, there is very little space left between theprojection lens and the target plane for a beamlet deflection system.The inventors recognized that the focal length should be of such limitedmagnitude that any deflector or deflector system should be locatedbefore the projection lens despite the evident occurrence of off-axisaberrations with such an arrangement.

The arrangement shown in FIGS. 1 and 2 of the deflection array 9upstream and projection lens arrangement 10 downstream furthermoreallows a strong focusing of beamlet 21, in particular to permit areduction in size (demagnification) of the beamlets of at least about100 times, and preferably about 350 times, in systems where eachprojection lens system focuses only one beamlet (or a small number ofbeamlets). In systems where each projection lens system focuses a groupof beamlets, preferably from 10 to 100 beamlets, each projection lenssystem provides demagnification of at least about 25 times, andpreferably about 50 times.

This high demagnification has another advantage in that requirements asto the precision of the apertures and lenses before (upstream of) theprojection lens arrangement 10 are much reduced. Another advantage ofthis arrangement is that the column length (height) of the overallsystem can be greatly reduced. In this respect, it is also preferred tohave the focal length of the projection lens small and thedemagnification factor large, so as to arrive to a projection column oflimited height, preferably less than one meter from target to electronsource, and more preferably between about 150 and 700 mm in height. Thisdesign with a short column makes the lithography system easier to mountand house, and it also reduces the effect of drift of the separatebeamlets due to the limited column height and shorter beamlet path. Thisarrangement, however, puts additional demands on the various componentsof the end module.

With an arrangement as described above, the main lens plane of theprojection lens system 10 is preferably located between the two plates13 and 14. The overall energy of the charged particles in the systemaccording to the embodiments described above is kept relatively low, asmentioned previously. For an electron beam, for example, the energy ispreferably in the range of up to about 10 keV. In this way, generationof heat at the target is reduced. However, with such low energy of thecharged particles, chromatic aberration in the system increases. Thealready mentioned relatively high electrostatic field in the projectionlens arrangement 10 may counteract this detrimental effect. A highelectrostatic field results in forming electrostatic lenses having a lowfocal length, so that the lenses have low chromatic aberration.

Furthermore, the final plate 14 of the lens system 10 according to thepresent invention may be made very thin to enable a small focal lengthwithout the focal plane being inside the lens. The thickness of plate 14is preferably within the range of about 50 to 200 μm.

As discussed above, electrostatic lenses used as the projection lenssystems discussed with reference to FIGS. 1 and 2, have to function inhigh electric fields. As mentioned before, this requires well definedplate processing provided with uniformly sized, uniformly pitched anduniformly shaped apertures as well as well defined alignment betweenplates. A person skilled in the art will readily understand that a riskassociated with such electrostatic lenses is an elevated chance ofelectrostatic discharges.

Such electrostatic discharges could not only be detrimental to thephysical structure of electrostatic lens components, but could alsonegatively effect the control and accuracy of the electron beamletspassing through the lenses.

Electrostatic discharges occur across exposed surfaces located in theapertures and at the edges of the apertures. Generally, such dischargesoccur in areas of local electrostatic field enhancement where theelectrostatic field strength is increased, often where there is largevariation in permittivity c. These areas of higher electrostatic fieldcan result sudden electrical discharges or “flashovers” resulting inlocal breakdown of insulating materials and distortion and deteriorationof the electrostatic lens.

FIGS. 3A-3C schematically depict three situations where such localelectrostatic field enhancement and increased risk of flashovers mayoccur. All figures schematically show a side view of a portion of anelectrostatic lens comprising a first electrode 31 and a secondelectrode 32. A voltage difference between the electrodes 31, 32 is suchthat field lines 33 of a generated electric field run substantiallyparallel to the surfaces of the electrodes 31, 32 that face each other.

In FIG. 3A, the second electrode 32 comprises a sharp protrusion 35. Asa result of the protrusion 35, especially if the protrusion isconducting, field lines 33 are bent resulting in an elevated electricfield around the protrusion 35, i.e. field lines 33 are located closerto each other, particularly at the sharp corners of the protrusion,indicating a higher electric field strength. These higher electricfields increase the chance of electrostatic discharges if the potentialof the second electrode 32 is below the potential of the first electrode31, i.e. electrode 31 is negative with respect to electrode 32. Thisoccurs when the electric field is high enough to pull electrons from thesecond electrode 32 and start a flow of electrons that will create aflashover.

In FIG. 3B, the first electrode 31 and the second electrode 32 areseparated from each other by means of an insulating structure 37, e.g. aspacer made of an insulating material. Due to an imperfection, the firstelectrode 31 and the insulating structure 37 are not entirely in contactwith each other along the upper surface of the insulating structure 37.This may be caused by manufacturing errors or tolerances, irregularitiesin the surface of the insulating structure or electrode, or chipping orrounding of the edge of the insulating structure. As a result, a smallvoid 38 (shown larger in the drawing for clarity) is present between theinsulating structure 37 and the first electrode 37, at the edge of theinsulating structure. If there is a significant difference inpermittivity between the insulating structure 37 and the permittivity inthe void 38 (i.e. the permittivity of air, or vacuum if the equipment isoperated in a vacuum chamber), the electric field in the void 38 may beconsiderably higher than in other areas between the two electrodes.

The average electric field in a gap between electrodes is the potentialdifference divided by the minimum electrode separation. Where theelectrodes are separated by insulating regions or materials having twodifferent dielectric constants, the electric field will be concentratedin the region or material with the lower dielectric constant. Theelectric field is enhanced in this region or material, i.e. greater thanthe average electric field between the electrodes. The dielectricconstant (k) or relative permittivity of a material is the ratio of itspermittivity ∈ to the permittivity of vacuum ∈o, so k=∈/∈o).

Since the dielectric constant/permittivity of the void is lower than theinsulating structure, the electric field is enhanced in the void. Thesmaller the void, the greater field enhancement. This higher electricfield increases the chance of electrostatic discharges if the potentialof the first electrode 31 is below the potential of the second electrode32. The imperfection schematically shown in FIG. 3B may be caused byedge chipping of other manufacturing problems.

Also in FIG. 3C, the first electrode 31 and the second electrode 32 areseparated from each other by means of an insulating structure 37. Inthis case, imperfections in the connection between the first electrode31 and the insulating structure 37, caused for example by irregularitiesin the surface of the insulating structure or the electrode. Theseresult in the formation of internal voids 39 (enlarged for clarity). Theinternal voids 39 can create the same problem as the edge voids 38,resulting in locally enhanced electric fields in the voids which mayresult in an elevated chance of electrostatic discharges if thepotential of the first electrode 31 is below the potential of the secondelectrode 32.

It has been recognized by the inventors that it is of importance tolower the electric field strength at so-called triple points, i.e. apoint at which there is a boundary between air/vacuum, conductor andinsulator, particularly where the triple point occurs adjacent to a morenegative or lower voltage conductive element. Such triple points arepresent in the arrangements schematically shown in FIGS. 3A-C. Byproviding measures that limit the electric field enhancement at triplepoints adjacent to a more negative or lower voltage electrode in theelectrostatic lens or electrostatic lens array, the performance of theelectrostatic lens or electrostatic lens array with respect towithstanding high electric fields without occurrence of electricaldischarge increases considerably. In embodiments of the invention, someof them discussed below, the electrostatic lens or electrostatic lensarray is suited to accommodate a field strength of over 10 V/μm up to 25V/μm, or in a higher range 25-50 V/μm.

FIG. 4 schematically shows a side view of a portion of an electrostaticlens array 40 according to a first aspect of the invention. Theelectrostatic lens array 40 comprises a first conductive plate 41 and asecond conductive plate 42. The distance between the first conductiveplate 41 and the second conductive plate 42 may lie in a range fromabout 100 to 200 μm. The first conductive plate 41 is arranged to beelectrically charged with a potential lower than the potential of thesecond conductive plate 42. The first conductive plate 41 is providedwith a plurality of first apertures 43. The second conductive plate 42is provided with a plurality of second apertures 44. The plurality ofsecond apertures 44 are substantially aligned with the plurality offirst apertures 43. The edges of the first apertures 43 facing thesecond apertures 44 are rounded. As a result, electrostatic dischargescaused by sharp edges, which may act in a similar way as a sharpprotrusion as schematically depicted in FIG. 3A, are avoided.

In the embodiment depicted in FIG. 4, not only the edges facing thesecond apertures 44, but also the edges facing away from the secondapertures 44 are rounded. Such an embodiment may be easier tomanufacture.

In the embodiment depicted in FIG. 4, an insulating structure 47 hasbeen arranged between the first conductive plate 41 and the secondconductive plate 42. The insulating structure 47 is formed such that itenables transmission of charged particle beamlets through the pluralityof first apertures 43 and the plurality of second apertures 44. Theinsulating structure 47 may be used to enhance the structural integrityof the electrostatic lens array. Furthermore, the insulating structure47 may serve the purpose of positioning the first conductive plate 41and second conductive plate 42 with respect to each other, acting as aspacer to control the distance between the two plates.

It must be understood, that although the embodiments described withreference to FIG. 4 relate to an electrostatic lens array 40, the sameembodiments may be used for a single electrostatic lens. In this case,instead of a plurality of first apertures and a plurality of secondapertures, only a single first aperture and a single second aperture areused, in which the second aperture is substantially aligned with thefirst aperture.

FIG. 5 schematically shows a side view of a portion of an electrostaticlens or electrostatic lens array according to a second aspect of theinvention. The electrostatic lens comprises a first conductive plate 51and a second conductive plate 52. The two conductive plates 51, 52 areseparated by means of an insulating structure 57. The insulatingstructure 57 may have a thickness of about 100-200 μm. The conductiveplates 51, 52 are each provided with one or more apertures (not shown),the one or more apertures of the first conductive plate 51 beingsubstantially aligned with corresponding one or more apertures in thesecond conductive plate 52. The first conductive plate 51 is arranged tobe provided with a potential that is lower than or more negative withrespect to the potential provided to the second conductive plate 52, asindicated in the diagram by the “−” sign next to plate 51 and the “+”sign next to plate 52.

The insulating structure 57 includes a first portion 57A and a secondportion 57B. The first portion 57A is in contact with the firstconductive plate 51, while the second portion 57B is in contact with thesecond conductive plate 52. The portions 57A, 57B of the insulatingstructure 57 are arranged such that the first portion includes anoverhanging portion 57C and the second portion has an indented orrecessed portion 58 formed in the wall of the insulator 57, forming agap between the overhanging portion 57C and the second conductive plate52. As a result, the difference in permittivity between the insulatingstructure 57 and the permittivity in the gap or indented portion 58 willresult in a proportionally higher electric field developing across thegap than across first portion 57A. A permittivity of the insulatingstructure 57 of two times or more than in the gap 58 is preferred, andof approximately four times or more is more preferred. For example, theinsulator may be constructed to glass with a permittivity approximately4.7 times the permittivity of a vacuum which occupies the gap 58.

Thus, a field reduction occurs adjacent the more negative plate 51 and afield enhancement occurs in the gap between the first portion 57A andthe more positive plate 52. This field enhancement is relatively smalland does not cause a problem because the second conductive plate 52 ismore positive with respect to the first conductive plate 51. Theslightly higher field in the gap will not be sufficient to pullelectrons out of the insulator 57 to initiate current flow in aflashover. In addition, the lower field adjacent to the more negativeplate 51 further reduces the chance of electrons being pulled out of theconductive plate 51 to begin a flashover.

If the insulating structure 57 comprises an imperfection creating a voidadjacent to plate 51 as shown in FIG. 3B (e.g. an edge chipping), theelectric field in the void will be increased to a much lesser degreethan would be the case with an ordinary shaped insulating structure,e.g. a structure as shown in FIG. 3B. The presence of a recessed portioncreating a gap adjacent to plate 52 as shown in FIG. 5 will result inonly a lesser field enhancement occurring in both the gap and the void.This lower electric field adjacent to the more negative plate 51 furtherreduces the chance of an electrostatic discharge. Note that voids mayalso occur due to surface imperfections in the plate 51, and the gapwill similarly address this problem.

It should be noted that forming a recessed portion between the secondportion 57B and the first conductive plate 51 is a less preferredsolution. Even though such a measure helps to elongate the surfacebreakdown path, it does not help to lower the electric field at thetriple point near the more negative electrode where an edge chippingvoid may form. On the contrary, such a recessed portion would have asimilar effect as the voids in FIG. 3C, i.e. an electric fieldamplification where an attenuation is desired. Furthermore, forming arecessed portion on both sides of an overhanging portion of theinsulating structure so that gaps are formed against both first andsecond conductive plates 51 and 52 is also a less preferred solution,for the same reason.

The height d_(r) and/or width W_(r) of the gap may be varied in view ofthe requirements of the electrostatic lens (array). In the embodimentshown, the height d_(r) of the gap 58 is half the distance D between thefirst conductive plate 51 and the second conductive plate 52. Thedistance D may lie in a range from about 100 to 200 μm, although othergap heights d_(r) are also possible, e.g. a height between 0.3-0.7 timesthe distance D between the plates. Preferably the height d_(r) of thegap is as large as possible. The higher the proportion of the electricfield that develops in the gap, the less likely it is that electronsfrom the more negative plate 51 will cause a flashover. However, the gapheight d_(r) may be limited by the need to maintain structural integrityof the remaining rim, i.e. the first portion 57A.

In an alternative embodiment, the gap may be located so that it is notadjacent to either of plates 51 or 52, with a rim formed on either sideof the gap adjacent to the plates. However, maintaining a rim on bothsides of the gap reduces the height of the gap, and the height is againfurther liited by the need to maintain the structural integrity of therims of the insulating structure.

FIG. 6A schematically illustrates a side view of a portion of anelectrostatic lens or electrostatic lens array according to a thirdaspect of the invention. The electrostatic lens comprises a firstconductive plate 61 and a second conductive plate 62. The two conductiveplates 61, 62 are separated by means of an insulating structure 67. Theconductive plates 61, 62 are each provided with one or more apertures(not shown), the one or more apertures of the first conductive plate 61being substantially aligned with corresponding one or more apertures inthe second conductive plate 62. The first conductive plate 61 isarranged to be provided with a potential that is lower than thepotential provided to the second conductive plate 62.

A surface of the insulating structure 67, i.e. the surface facing thefirst conductive plate 61, is provided with a conductive layer 66.Because the conductive layer 66 is in contact with the first conductiveplate 61, it will be have essentially the same electrical potential. Ifvoids 69 are present between the insulating structure 67 and the firstconductive plate 61 as described in FIG. 3C, the conductive layer 66will be formed on the top surface of the insulating structure 67 overthe voids as shown in the magnified view of the electrostatic lensschematically depicted in FIG. 6B. The conductive layer 66 may also fillthe voids, depending on the size of the voids, the thickness of theconductive layer 66, and process used to form the conductive layer. Theconductive layer 66 now forms the interface between the conductive plate61 and the insulating structure 67, and any voids 69 between theconductive layer 66 and conductive plate 61 and there will beessentially no electric field present in these voids. Hence, flashoverdue to void-related effects will be minimized Note that the voids mayalso occur due to surface imperfections in the first conductive plate61, and the conductive layer 66 will similarly address this problem.

The conductive layer may be deposited onto the surface of the insulatingstructure 67 by a suitable depositing technique such as sputtering. Theconductive layer may comprise chromium or tantalum or other suitablematerials. Furthermore, the insulating structure may comprise aborosilicate glass, e.g. Pyrex, or suitable insulating materials. Aborosilicate glass is able to withstand high electric fields, andchromium and tantalum adhere easily to a borosilicate glass, which makessuch a combination of materials advantageous. Borosilicate glass is alsoa suitable insulating material because it can be shaped into a desiredformat, e.g. by means of a method of manufacturing that will bediscussed later, and through holes can be manufactured relatively easyin a plate comprising such a material, e.g. by means of power blasting.

FIG. 7 schematically shows a side view of a portion of an electrostaticlens array combining different aspects of the invention. In theembodiment shown, the insulating structure comprises portions forming agap 78 as discussed with reference to FIG. 5, and a surface of theinsulating structure facing the first conductive plate 71 is providedwith a conductive layer 76.

It should be understood that the different aspects of the invention mayeach be used alone, in combination with one or more other aspects of theinvention to reduce the chance of electrostatic discharges.

An insulating structure as discussed with reference to FIG. 4 may bemanufactured as follows. First, a first insulating plate is provided. Inthe first insulating plate, at least one first through hole ismanufactured. The first through hole has a first diameter. Then, asecond insulating plate is provided. In the second insulating plate atleast one second through hole is manufactured, with a second diameterlarger than the first diameter. Finally, the first insulating plate andthe second insulating plate are combined to form an insulating structurewith the apertures aligned such that the insulating structure has atleast one aperture formed by the first and second through holes.

The second insulating plate may be of a different material than thefirst insulating plate. Alternatively, the second insulating plate maybe of the same material as the first insulating plate. The insulatingplates being of the same material has the advantage that discontinuitiesin electrical and other properties at the boundary surface between thefirst and second insulating plates will be minimized.

FIG. 8A schematically shows a top view of an electrostatic lens arrayaccording to an embodiment of the invention. FIG. 8B schematically showsa side view of a portion of the electrostatic lens array of FIG. 8A. Inthis embodiment, an insulating structure 97 arranged between a firstconductive plate 91 provided with a plurality of first apertures 93 anda second conductive plate 92 provided with a plurality of secondapertures 94 takes the form of a plurality of elongate bars or struts.The bars are arranged such that multiple openings formed by theplurality of first apertures 93 and the plurality of second apertures 94are present between successive bars.

In FIG. 8A, the bars are represented by the black areas 83. Areas 81, 82correspond with areas provided with openings. In this embodiment, area82, in FIG. 8a represented by the widely spaced dashed areas, isprovided with openings that are not used for providing a path for acharged particle beam. On the other hand, area 81, in FIG. 8Brepresented by the densely dashed areas, provides openings that servethe purpose of providing a path for a charged particle beam. Theopenings in area 82 are present to prevent side and/or edge effects. Dueto the presence of the openings in area 82, the openings in area 81experience a substantially identical electrical environment.

FIG. 8B shows an embodiment in which edges of first apertures 93 in thefirst conductive plate 91 are rounded, a gap 98 is provided between aportion of the insulating structure 97 and the second conductive plate92, and the insulating structure 97 is provided with a conductive layer96 at a surface facing the first conductive plate 91. It must be notedthat it is also possible to any of the aspects of the inventiondiscussed with reference to FIGS. 4-7, either singly or in variouscombinations, with an insulating structure in a bar-like format as shownin FIG. 8B.

The invention has been described by reference to certain embodimentsdiscussed above. It will be recognized that these embodiments aresusceptible to various modifications and alternative forms well known tothose of skill in the art.

The invention claimed is:
 1. An electrostatic lens comprising: a firstconductive plate provided with a first aperture; a second conductiveplate provided with a second aperture, the second aperture beingsubstantially aligned with the first aperture; a voltage supply forsupplying a first voltage to the first conductive plate and a secondvoltage to the second conductive plate, the second first voltage beinglower than the first second voltage with respect to intended function ofthe lens; and an insulating structure for separating the firstconductive plate from the second conductive plate; wherein theinsulating structure comprises a first portion in contact with the firstconductive plate and a second portion in contact with the secondconductive plate, the first portion having an overhanging portioncontacting said first conductive plate and the second portion having anindented portion at an edge of the insulating structure, and wherein agap is formed between the overhanging portion and the second conductiveplate, and wherein a surface of the insulating structure facing thefirst conductive plate is provided with a conductive layer in electricalcontact with the first conductive plate for limiting electric fieldenhancement between the first conductive plate and the insulatingstructure.
 2. The electrostatic lens of claim 1, wherein thepermittivity of the gap is lower than the permittivity of the insulatingstructure.
 3. The electrostatic lens of claim 2, wherein thepermittivity of the gap is at least four times lower than thepermittivity of the insulating structure.
 4. The electrostatic lens ofclaim 1, wherein, in operation, the electric field strength in the gapbetween the overhanging portion of the insulating structure and thesecond conductive plate is greater than the electric field strengthacross the second portion of the insulating structure.
 5. Theelectrostatic lens of claim 1, wherein the first portion and the secondportion of the insulating structure are of equal thickness.
 6. Theelectrostatic lens of claim 1, wherein the first portion and the secondportion of the insulating structure comprise separate structures whichare bonded together.
 7. The electrostatic lens of claim 1, wherein asurface of the insulating structure facing the first conductive plate isprovided with a conductive layer in electrical contact with the firstconductive plate for limiting electric field enhancement between thefirst conductive plate and the insulating structure.
 8. Theelectrostatic lens of claim 7 1, wherein the conductive layer isdeposited on a surface of the insulating structure using a depositiontechnique.
 9. The electrostatic lens of claim 7 1, wherein saidconductive layer comprises chromium or tantalum.
 10. The electrostaticlens of claim 1, wherein a distance between the first conductive plateand the second conductive plate lies in a range from about 100 to 200μm.
 11. The electrostatic lens of claim 1, wherein the lens is capableof withstanding a field strength of over 10 V/μm without flashover, andmore preferably within a range from 25-50 V/μm without flashover. 12.The electrostatic lens of claim 1, wherein the insulating structurecomprises borosilicate glass.
 13. An electrostatic lens array comprisinga plurality of electrostatic lenses according to claim
 1. 14. Theelectrostatic lens array of claim 13, wherein the insulating structuretakes the form of an insulating plate provided with at least one thirdaperture, the at least one third aperture being arranged such that aprojection of its side walls circumscribes multiple first apertures andsecond apertures.
 15. The electrostatic lens array of claim 13, whereinthe insulating structure takes the form of a plurality of elongatedbars, such that between successive bars multiple transmission pathsformed by first apertures and second apertures are present.
 16. Acharged particle beamlet lithography system comprising: a source ofcharged particles for producing a beam of charged particles; an aperturearray for producing a plurality of beamlets from the charged particlebeam; a beamlet conditioning system for conditioning the plurality ofbeamlets in accordance with a pattern; and the electrostatic lens arrayaccording to claim 13 for focusing the plurality of beamlets.
 17. Thecharged particle beamlet lithography system of claim 16, wherein saidelectrostatic lens array is positioned between said aperture array andsaid beamlet conditioning system.
 18. The charged particle beamletlithography system of claim 16, wherein said charged particle beamletlithography system further comprises a support unit for supporting asubstrate to be patterned and said electrostatic lens array ispositioned downstream of said beamlet conditioning system for focusing aplurality of conditioned beamlets on a target surface of the substrate.19. A method of manufacturing an insulating structure for use in anelectrostatic lens, the method comprising: providing a first insulatingplate; manufacturing at least one first through hole in said firstinsulating plate, said at least one first through hole having a firstdiameter; depositing a conductive layer on a surface of the firstinsulating plate; providing a second insulating plate; manufacturing atleast one second through hole in said second insulating plate, said atleast one second through hole having a second diameter, said seconddiameter being larger than said first diameter; and combining the firstinsulating plate and the second insulating plate to form an insulatingstructure with the at least one first through hole and the at least onesecond through hole aligned such that said insulating structure has atleast one aperture formed by said at least one first and second throughholes, and such that the conductive layer is provided on a surface ofthe insulating structure.
 20. The method of claim 19, wherein the firstinsulating plate and the second insulating plate are made of the sameinsulating material.
 21. The method of claim 19, wherein at least one ofthe first insulating plate and the second insulating plate comprisesborosilicate glass.
 22. The method according to claim 19, wherein atleast one of the manufacturing the at least one first through hole andthe manufacturing the at least one second through hole is performed bypower blasting.
 23. An insulating structure for use in an electrostaticlens manufactured according to the method of claim
 19. 24. Anelectrostatic lens array comprising: a first conductive plate providedwith a plurality of first apertures; a second conductive plate providedwith a plurality of second apertures, the second apertures beingsubstantially aligned with the first apertures; a voltage supply forsupplying a first voltage to the first conductive plate and a secondvoltage to the second conductive plate, the first voltage being lowerthan the second voltage with respect to intended function of the lens;and an insulating structure for separating the first conductive platefrom the second conductive plate, wherein the insulating structure isprovided with a third aperture, the third aperture comprising sidewalls, wherein the third aperture is arranged such that a projection ofthe side walls circumscribes multiple of said first apertures and saidsecond apertures, and wherein the insulating structure comprises a firstportion in contact with the first conductive plate and a second portionin contact with the second conductive plate, the first portion having anoverhanging portion contacting said first conductive plate and thesecond portion having an indented portion at an edge of the insulatingstructure, and wherein a gap is formed between the overhanging portionand the second conductive plate.
 25. Electrostatic lens array accordingto claim 24, wherein the surface of the insulating structure facing thefirst conductive plate is provided with a conductive layer in electricalcontact with the first conductive plate for limiting electric fieldenhancement between the first conductive plate and the insulatingstructure.
 26. The electrostatic lens array of claim 25, wherein theconductive layer is deposited on a surface of the insulating structureusing a deposition technique.
 27. The electrostatic lens array of claim25, wherein said conductive layer comprises chromium or tantalum. 28.The electrostatic lens array according to claim 24, wherein the edges ofthe first apertures facing the second apertures are rounded.
 29. Theelectrostatic lens array of claim 24, wherein the permittivity of thegap is lower than the permittivity of the insulating structure.
 30. Theelectrostatic lens array of claim 24, wherein, in operation, theelectric field strength in the gap between the overhanging portion ofthe insulating structure and the second conductive plate is greater thanthe electric field strength across the second portion of the insulatingstructure.
 31. The electrostatic lens array of claim 24, wherein thefirst portion and the second portion of the insulating structure are ofequal thickness.
 32. The electrostatic lens array of claim 24, wherein adistance between the first conductive plate and the second conductiveplate lies in a range from about 100 to 200 μm.
 33. The electrostaticlens array of claim 24, wherein the lens is capable of withstanding afield strength of over 10 V/μm without flashover.
 34. The electrostaticlens array of claim 24, wherein the insulating structure comprisesborosilicate glass.